In many automated mail sorting systems, stacks of flat mail pieces such as letters or flats are singulated by a feeder that advances the stack against a pickoff belt, which pickoff belt feeds individual mail pieces from the end of the stack to an opposed belt conveyor. One such feeder is disclosed in U.S. Pat. No. 6,679,491 issued Jan. 20, 2004 to Luebben et al., the contents of which are incorporated herein for all purposes. The singulated stream of mail pieces is conveyed past a scanner such as an OCR (optical character reader) or bar code reader (BCR) that captures an image of destination data from the address side of the mail piece. The image data is decoded and used to sort the mail pieces for delivery.
In order to process the large volume of mail received daily by the U.S. Postal Service, the feed rate of such feeders is normally in the thousands of mail pieces per hour. Operating at such rates, a feeder will occasionally pick off more than one mail piece from the end of the stack of mail pieces, resulting in a “double” or multiple feed where two or more overlapping mail pieces are presented to the imaging camera at the same time. Such multiple feeds result in mis-sorted mail and can also cause jams in downstream processing equipment, resulting in costly delays.
U.S. Pat. No. 6,817,610, issued Nov. 16, 2004 to Rompe, the disclosure of which is incorporated herein for all purposes, discloses a method and apparatus for detecting overlapped mail pieces as the mail pieces are conveyed on edge over a line camera which scans the bottom edges of the mail pieces. Features of the scanned image are processed using statistical techniques and compared to models of known mail piece configurations to determine the probability that the scanned image data represents overlapping mail pieces. This method relies solely on a view obtainable from the bottom of the mail pieces. There remains a need for further improvement in doubles detection in automated mail processing systems.